1. Field of the Invention
The present invention relates to a display and an image displaying method, and, more particularly, to a display and an image displaying method, which carry out gradation expression by a sub field system and sequentially output and display data line by line in each sub field.
2. Description of the Related Art
Recently, flat and light panel displays which uses a liquid crystal or plasma and which have less screen distortion and are less susceptible to the influence of geomagnetism are becoming popular and are replacing conventional brown tube (CRT) displays. Particularly, attention is paid to the plasma display, which can be designed into a large panel relatively easily, as a display for video images.
In general, because the plasma display has a difficulty in displaying an intermediate gradation level between emission and non-emission, the intermediate gradation level is displayed using the so-called sub field system. The sub field system separates the time interval of one field into plural sub fields, assigns specific emission weights to the sub fields and controls the emission and non-emission of each sub field, thereby displaying gradation levels of the luminances of one field.
In the address-sustain separation system that is a leading system for plasma displays at present, one sub field consists of a reset period for initializing the states of discharge cells, an address control period for controlling ON/OFF of the discharge cells, and a sustain period for determining the amount of emission. Those periods are controlled by control pulses. The time intervals of those control pulses cannot be made shorter than a predetermined time period in order to realize stable emission control.
In the address control period, as addressing is carried out based on data that controls ON/OFF of the discharge cells line by line, a high-resolution panel requires a longer addressing time due to the large number of lines to be scanned. This raises such a problem that the number of sub fields providable in one field period is limited or sufficient luminance cannot be acquired. In case where a high-definition panel having a vertical resolution of 1000 lines is constructed using a display panel which requires 2 xcexcs per line in the address control process, for example, the address control period of 2 ms (=2 xcexcsxc3x971000 lines) per sub field is needed. In general, displaying a video signal without degradation requires gradation of about 256 gradation levels (8 bits). If eight sub fields are to be provided in one field period of about 16.6 ms, there hardly is a time left to be assigned to the sustain period. Because one field period is mostly assigned to the address control period for each sub field, it is not possible to secure a sufficient sustain period that contributes to emission of the panel.
Japanese Patent Laid-Open No. 24628/1999 discloses the scheme that shortens the address control time by performing skip scanning for sub fields corresponding to lower bits and the system that performs a writing operation by simultaneously selecting two scan electrodes instead of using skip scanning. This document however discloses no specific signal generating schemes.
Each line of a video signal is data sampled in the vertical direction of one screen. At the time of thinning sampling data in skip scanning, the vertical resolution should be reduced to a half beforehand in order to decrease the cyclic interference. This lowers the vertical resolution by a half, providing a resolution-dropped image.
It is known that when sampling data is thinned without reducing the vertical resolution beforehand, a signal with a high-frequency component is converted to a DC signal or a signal of a lower frequency due to the cyclic interference, thus significantly degrading the image quality.
When the number of sub fields is limited to, for example, six sub fields for 64 gradation levels in order to achieve high luminance, the gradation cannot be displayed sufficiently, which makes it difficult to realize a high-image-quality display.
The conventional plasma displays that, unlike a CRT display, do not have the gamma characteristic have a tendency of making the display gradation on the low-luminance side rougher. It is said that if the gradation step in the vicinity of the black level is improved to the level of a CRT display, a gradation range from 10 bits (1024 gradation levels) to 12 bits (4096 gradation levels) is needed. Even with panels having a low vertical resolution (a smaller number of lines), therefore, the conventional displays employ a scheme of increasing the number of display gradation levels in a pseudo fashion before displaying by dithering, an error diffusion process or the like in order to make up the deficiency of the number of display gradation levels.
The systems, such as dithering and the error diffusion process, that display an image after increasing the number of display gradation levels in a pseudo fashion ensure pseudo display of an average luminance by enabling or disabling the minimum gradation steps. When the number of the minimum gradation steps is xe2x80x9c1xe2x80x9d, for example, a gradation level of 0.5 is expressed in a pseudo fashion by alternately enabling and disabling the minimum gradation step and a finer intermediate gradation level can be expressed equivalently by changing the ratio of the enableness and disableness.
It is known that while the use of the pseudo intermediate gradation can display a greater number of gradation levels than the actual gradation levels in a pseudo fashion, the enabling/disabling pattern of the minimum gradation step becomes noticeable as dot noise.
In the gradation display based on the sub field system, the gradation of the minimum gradation step is equivalent to the amount of emission of the least significant sub field. As mentioned above, the conventional plasma displays that, unlike a CRT display, do not have the gamma characteristic have a tendency of making the display gradation on the low-luminance side rougher.
In case where the pseudo intermediate gradation is adapted, therefore, interference is likely to be noticeable which is originated from dot noise that is produced when the gradation between the black level and the gradation of the minimum gradation step at which the least significant sub field is enabled is expressed in a pseudo fashion.
In case where data of the least significant sub field is made identical between upper and lower lines in the system disclosed in Japanese Patent Laid-Open No. 24628/1999, the dot area of the dot noise becomes twice as large, significantly deteriorating the image quality.
Accordingly, it is an object of the invention to provide a display technique which can shorten the address control period in accordance with the required luminance and assign the produced extra time to an improvement of the image quality, such as the luminance, gradation and pseudo contour.
It is another object of the invention to provide a display technique which has a high luminance or an excellent gradation characteristic and can shorten the address control period while keeping the effect of suppressing the conventional pseudo contour interference.
The invention aims at providing a display and an image displaying method, which are provided with a signal processing circuit for processing sub field data by referring to signals of plural lines that are to be made common in a lower sub field in such a way that the degradation of the image quality occurs less and predetermined sub field data becomes identical, and which limit the amount of resolution information of a displayed image as needed by positively using the human visual characteristics or the statistical property of video images to thereby improve the general image quality.
The invention also aims at providing a display and an image displaying method which can set error-diffusion originated dot noise near the black level to the level provided by the prior art.
To achieve the objects, the invention uses the following means.
The invention uses a signal processing circuit which processes data of lower sub fields, excluding the least significant sub field, by referring to signals of plural lines, so that predetermined sub field data becomes identical.
Further, the interference originated from dot noise in pseudo intermediate gradation display is set to the same level as provided by the prior art by controlling the least significant sub field dot by dot.
Furthermore, an average value f0 of plural lines to be referred to is computed and an error diffusion process is executed based on the average value f0.
Specifically, the average value f0 is separated into a display effective bit (f0M) and a non-display lower bit (f0L) and the non-display lower bit is added to a residual error component (f0Exe2x80x2) which could not be displayed with the available display pixels. When the added value reaches the amplitude that is equivalent to the display effective bit, the display effective bit is increased to update the undisplayable residual error component.
According to the invention, it is determined if an average luminance level which is displayable by a combination of minimum gradation steps of the reference lines is present in the residual error component (f0E), and, if such an average luminance level is present, the minimum gradation steps of the reference lines are changed to ensure intermediate gradation display. The residual error component is updated by this intermediate gradation display.
Only when the least significant bit (LSB) data of the display effective bit (f0xe2x80x2) is xe2x80x9c0xe2x80x9d, an intermediate gradation level is displayed by a combination of the least significant bit steps of the reference lines.
Moreover, the residual error component is diffused to the adjoining pixels on the display screen.